I
NEVER start with a slip knot because I don't want to have to pick
it out. Just hold your two ends on the back of the needle and start
your cast on.

I always cast
on with two ends (both from the same ball OR two balls.)

In the picture
I have used two different colors to do the Twisted German Cast On.
Because I am working TOWARD the balls, I can cast on an infinite
number of stitches without any tedious measuring of my "long"
tail.